EP3230564A1

EP3230564A1 - Device for post-treating exhaust gases of a combustion engine

Info

Publication number: EP3230564A1
Application number: EP15804173.1A
Authority: EP
Inventors: Nils Matthess; Thierry Bertin; Jean Florent GENIES; Thomas Le Tallec
Original assignee: PSA Automobiles SA
Current assignee: PSA Automobiles SA
Priority date: 2014-12-10
Filing date: 2015-11-19
Publication date: 2017-10-18
Anticipated expiration: 2035-11-19
Also published as: FR3029969A1; EP3230564B1; WO2016092170A1; CN107002533B; CN107002533A

Abstract

The invention relates to a device for the post-treatment of the exhaust gases of a combustion engine (1), characterized in that it comprises, from upstream to downstream: an oxidation catalyst body DOC (3) or an LNT NOx trap (3') ; an outlet (41) of a means (4) for introducing a reducing agent or reducing agent precursor for the selective catalytic reduction SCR of nitrogen oxides; a particle filter part (7) provided with a catalyst coating for the selective catalytic reduction SCRF of nitrogen oxides NO _x; and a part (8) for treating ammonia leaks.

Description

EXHAUST GAS POST-TREATMENT DEVICE

OF A COMBUSTION ENGINE

[001] The invention relates to means for treating pollutants from the exhaust gases of combustion engines.

[002] The pollutant emissions of combustion engines fitted to motor vehicles are regulated by standards. Regulated pollutants are, depending on the combustion engine technology considered, carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx, ie NO and NO _¾) and particles (PM), which are formed during combustion of the fuel in the combustion chamber and then emitted to the exhaust.

[003] It is known to employ a number of pollution control means in the exhaust line of combustion engines to limit the emissions of regulated pollutants. An oxidation catalyst allows the treatment of carbon monoxide, unburned hydrocarbons, and under certain conditions nitrogen oxides; a particulate filter can be used for the treatment of soot particles.

[004] This type of device is generally referred to as the "after-treatment" device for the exhaust gases. [005] To meet pollution control standards on nitrogen oxide (NOx) emissions, a specific post-treatment system can be introduced into the exhaust line of vehicles, including vehicles equipped with diesel engines. For the treatment of nitrogen oxides (NOx), selective catalytic reduction (SCR) technologies for Selective Catalytic Reduction (English) are known, which consist in reducing NOx by introducing a reducing agent (or a precursor of such a reducing agent) in the exhaust gas by catalyzed reactions. It may for example be a urea solution, the decomposition of which will make it possible to obtain ammonia which will serve as a reducing agent, but also of a reducing agent or a precursor of such a reducing agent. gaseous form. In the remainder of the present document, a "reducing agent" will generally be used to designate a reducing agent or a reducing agent precursor. The reducing agent generated makes it possible to reduce the nitrogen oxides by reaction in an SCR catalyst, that is to say a substrate carrying a catalytic impregnation capable of promoting the reduction of NOx by the reducing agent.

[007] European standards, in particular, tend to become more and more severe. And solutions to reduce pollutant emissions at the end of the exhaust line to meet current standards will prove insufficient given the changes in standards planned beyond 2017.

[008] Indeed, the first step of the standard, Euro 6b (effective in September 2014) led car manufacturers to choose between different options to reduce NOx emissions more specifically: - the reduction of NOx "to the source ", at the level of the actual operation of the engine, via technologies of the exhaust gas recycling type in the engine, recycling also known as EGR technology according to the acronym for the English term corresponding to" Exhaust Gas Recirculation "high and low pressure, for example ; - the reduction of NOx at the exhaust line via a sequential catalytic treatment technology called "NOx trap"; the reduction of NOx at the level of the exhaust line also, via a continuous treatment technology called "selective catalytic reduction" as briefly described above (SCR); even by cumulating many of these solutions.

[009] If these solutions make it possible to satisfy this first step in the evolution of the standard (Euro6b), they are not necessarily capable of satisfying the second stage which promises to be even more severe (Euro 6c, entry into force foreseen in September 2017), with pollutant measurements on a new rolling cycle called "WLTC" (for "Worldwide Harmonized Light Vehicle Test Cycle" in English, or harmonized test cycle for light vehicles in French), containing more transient phases than the current approval cycle (known as "MVEG" for Motor Vehicle Emissions Group in English, or group of emissions for motorized vehicles in French), but also off-cycle measures (called "RDE" for Real Driving Emission or emissions in conditions driving practices) should be introduced.

[0010] In particular, to meet the risks of excessively high NOx emissions outside the cycle, various technological solutions and architectures can be envisaged. They have their advantages and disadvantages. But the most efficient nitrogen oxide treatment technology is selective catalytic reduction (SCR) because it is efficient in more extended temperature and gas flow ranges than a NOx trap, the other solution post-processing. [001 1] In addition, there are constraints of implantation of the post-processing device. In fact, in general, the catalytic systems used are all the more effective as the temperature of the exhaust gases passing through them is high (to a certain extent). They will then start all the more quickly after starting the engine as the temperature of the exhaust gas increases rapidly. It is therefore advantageous to implement the after-treatment devices as close to the engine, that is to say closer to the exhaust manifold, under hood, even though this environment is generally very crowded. The post-processing devices must therefore be as compact as possible without affecting their performance. Throughout the present text, the terms "upstream" and "downstream" are understood as a function of the general direction of flow of the exhaust gases in the exhaust line integrating the post-processing units, since the engine to the end cannula of the exhaust line.

It is, for example, known from the patent application WO 201 1/089330 a post-processing device grouping in the same envelope several bodies that will be successively traversed by the exhaust gas. It is proposed, in particular, a series of organs comprising upstream downstream: - an oxidation catalyst, - a urea reducing agent injector, - a mixer whose role is to intimately mix the droplets of urea injected into the envelope traversed by the gases, so as to decompose to ammonia as homogeneously as possible over the entire cross section of the envelope, - an organ SCR, - a particulate filter (called FAP by the after). It also proposes an alternative, consisting of replacing the SCR member and the FAP, with a FAP which is impregnated with a NOx reduction catalyst and which thus fulfills both the soot filter function and the reduction of the NOx (called SCRF later).

However, a dedicated member SCR as described in this document may not start early enough for reasons of adverse thermal, especially in urban driving conditions during which the temperatures in the exhaust line are quite low . However, it is precisely during this type of urban driving that changes in the European standard (in particular) will become binding in terms of reducing NOx emissions.

In addition, additional constraints arise when the motor vehicle is a vehicle called "heavy" (more than 1500 kg), whether a vehicle for private or utility type. Indeed, under the same driving conditions as less heavy vehicle, the "heavy" vehicle will have higher temperature conditions in the exhaust to manage, and larger amounts to deal with NOx generated in the engine. To compensate for these higher NOx emissions, the amounts of reducing agent to be injected into the exhaust line (for example urea decomposing to ammonia) will have to be larger too, since these quantities are dictated by the stoichiometry of the NOx reactions with ammonia. The higher temperatures of the gases at the engine outlet also favor the thermo-desorption of the ammonia stored in the SCR (and / or SCRF) members, and can furthermore contribute to the degradation of their active catalytic phases which can induce a decrease in their storage capacity in ammonia. The combination of higher temperatures and a higher amount of urea (or ammonia) to be injected on the line induces an increased risk of ammonia emissions that would not react at the end of the exhaust line. . Ammonia leaks at the end of the exhaust line are smelly, and can be inconvenient, especially if the vehicle is in a confined space of closed parking type. The invention then aims to design a post-treatment of the engine exhaust gas that overcomes the aforementioned drawbacks. One of its aims is to improve existing systems to meet higher standards for pollutant emissions, and more particularly for NOx emissions under unstabilized rolling conditions of the urban rolling type and / or an extended temperature range, while limiting as much as possible leakage of reducing agent NOx unreacted end of the exhaust line. Advantageously, it also aims to obtain a postprocessing device that is more efficient and which remains, in addition, compact.

The invention firstly relates to a device for post-treatment of the exhaust gas of a combustion engine, which comprises, from upstream to downstream:

an oxidation catalyst member (also called DOC) or a NOx trap member (also called LNT for "Lean NOx Trap", acronym for poor NOx trap, which is an organ for storage and removal of NOx) ;

a mouth of a reducing agent introduction means or a precursor of a reducing agent for the selective catalytic reduction of the SCR nitrogen oxides;

a particulate filter element provided with a catalytic selective catalytic reduction coating SCRF of NOx nitrogen oxides; an organ for treating ammonia leaks.

This post-processing device architecture has proved extremely favorable in several aspects.

It improves the performance of the device, particularly with regard to the reduction of NOx in the least favorable conditions, namely, as mentioned above, in urban driving conditions (where the temperature of the exhaust gases) exhaust remains lower than a road or motorway type taxi); even conditions of aggressive driving type, with high flow rates of exhaust gas to be treated in jerks. And these very interesting results were obtained by a single member SCR integrated in the particulate filter, which is particularly advantageous, especially in terms of compactness.

Preferably, the pollution control system comprises only the organs listed above (except for any additional devices not directly involved in the depollution, but rather to its measurement, such as sensors for example).

It was also shown that this architecture made it possible to limit as much as possible the ammonia discharges / leaks at the end of the exhaust line (also referred to in English as "NH ₃ slip"). ammonia from either the reducing agent injected upstream of the unreacted SCR catalyst carrying member or formed during the purging of the LNT) by the addition of a these ammonia leaks. This is very advantageous, especially for heavy vehicles that require the injection of a larger amount of reducing agent, with an increased risk of ammonia release at the end of the line.

The device according to the invention will treat the gaseous and particulate pollutants as they pass through the depollution organs: they thus first enter the first "brick" constituted, according to a first embodiment , oxidation catalyst, where CO and HC are oxidized to water (H ₂ 0) and carbon dioxide (CO ₂ ). Out of this first DOC brick products of the oxidation of CO and HC namely H ₂ 0 and C0 ₂ , as well as oxides of nitrogen and particles. The residual NOx, and particles enter the SCRF brick, which will reduce the NOx by NH ₃ and remove the particles by storing them before burning them during regenerations. In the other embodiment where the DOC oxidation catalyst is replaced by a NOx trap, the presence, in "upstream" of the SCR member, NOx trap, further guarantees that a part at least NOx will be treated before entering SCRF. It is recalled that the NOx traps are sequential NOx treatment systems, with a lean NOx storage (excess oxygen), then a destocking and a NOx reduction in a rich mixture (excess of reducing agents): - in a mixture poor, NOx are stored as nitrates at active sites (usually in the form of single or mixed oxides such as oxide or barium aluminate); - In rich mixture, the NOx are destocked and reduced in N ₂ by the reducing agents (CO, HC and H ₂ ), it is called purge NOx. To ensure this function of trapping and reducing NOx, the active phase of the catalyst generally comprises noble metals, of the Pt, Pd type, ensuring the oxidation of NO to NO ₂ and CO / HC, thus playing the same role. a conventional oxidation catalyst, rhodium to ensure the reduction of NOx rich mixture, and a simple or mixed oxide for storing NOx lean mixture. The operation of the NOx trap therefore requires periodic wealth swings, the trap storing up to NOX saturation, and then being "purged" periodically by a change in richness.

Preferably, the length of the SCRF member is at least 100 mm, in particular between 100 and 250 mm, preferably between 120 and 180 mm. Preferably, the total length of the particulate filter is at most 250 mm, preferably between 120 and 180 mm. The invention thus preserves the compactness of the assembly.

Preferably, the total length between the inlet of the oxidation catalyst member or of the NOx trap and the outlet of the particulate filter is at most 450 mm, in particular at most 400 mm, from preferably between 280 and 380 mm.

Finally, the possible excess of ammonia is treated by the ad hoc treatment unit at the downstream end of the post-treatment device.

According to a first variant, the DOC oxidation catalyst member or the NOx trap, the mouth, and the particulate filter member provided with a catalytic selective catalytic reduction coating SCRF NOx nitrogen oxides are grouped in a single envelope, and the ammonia leak treatment member is disposed outside said single envelope. It is thus possible to distribute the various organs of the post-processing device, depending on the space available under the hood: thus, the bodies except the one treating the ammonia leak remain grouped under hood engine output, while the ammonia leak treatment unit can be deported further downstream on the exhaust line, and thus be in the underbody area of the vehicle.

Optionally, the action of the ammonia leak treatment unit is completed (or even replaced) by an ammonia leak treatment coating integrated with the particulate filter, preferably in its downstream portion.

According to another variant, said organs and mouth are grouped in a single envelope.

This architecture preserves the compactness of the assembly, which is contained in a single envelope, and which can thus be advantageously housed as close to the exhaust manifold at the engine output on the exhaust line, and in particular under hood. Preferably, the ammonia leak treatment unit is a catalyst for treating ammonia leakage by oxidation of ammonia to NOx and then reducing said NOx to nitrogen, which can be used in the following. of this text refer to ASC. "ASC" is the acronym for "Ammonia Slip Catalyst" or catalyst for ammonia leaks in French. Alternatively, the ammonia leak treatment unit may be a catalyst for cleaning ammonia leakage by ammonia oxidation, which may be referred to below as CUC in the rest of this text. "CUC" is the acronym for the term "Clean-Up Catalyst" or cleaning catalyst for the treatment of ammonia leaks. This type of catalyst only oxidizes ammonia to NOx.

Advantageously, the oxidation catalyst member may also comprise an adsorber material of nitrogen oxides, also called PNA which is the acronym for the expression "Passive NOx Adsorber".

The role of a PNA type material is to be able to store during the cold phases the nitrogen oxides emitted by the engine, as the bodies catalyzing the reduction of NOx (the SCR member and the particulate filter with SCRF catalytic coating) are not yet functional. Indeed, it is necessary to wait 180 to 200 ° C to be able to inject the reducer (urea) in the exhaust line and form the ammonia which will then convert the NOx. With NH ₃ "pre-stored" in the SCR coating, the conversion of NOx can take place a few tens of degrees before (at about 140 ° C). The NAP works by storing NOx "cold" (thanks, in particular, to the addition, in the "classical" impregnation of an oxidation catalyst, of simple or mixed oxides with a basic character such as, for example , oxides of cerium or barium) before returning them to higher temperature when the SCR is fully operational (between 200 and 300 ° C). In order to ensure proper operation of the PNA, purge steps are provided to clean its surface which has been sulfated over time in a known manner.

Preferably, the post-treatment device according to the invention comprises a mixing device for mixing the exhaust gas and the reducing agent and / or converting the precursor into a reductant between the mouth of the feed introduction means. reductant or precursor of a reducing agent for the selective catalytic reduction of nitrogen oxides and the selective catalytic reduction catalytic converter of nitrogen oxides integrated in the particulate filter SCRF.

The post-treatment device may also include a NOx sensor between the particulate filter member provided with a selective catalytic reduction catalyst coating SCRF NOx nitrogen oxides and the leak treatment member. ammonia, and optionally another NOx sensor upstream of the DOC oxidation catalyst member / LNT NOx trap, and / or another NOx sensor downstream of the ammonia leak treatment member. The "upstream" sensor, upstream of the DOC catalyst or the NOx trap, can also be replaced by modeling if necessary.

Preferably, the oxidation catalyst member has a catalyst whose amount of noble metals is adjusted so as to obtain at the outlet of the organ exhaust gas whose ratio N0 ₂ / NOx is equal or similar of 0.5 (we understand by "neighbor" a variation of for example +/- 15% around this value).

Preferably, the catalyst of the particulate filter is based on zeolite (s) exchanged (s) copper. Indeed, this type of catalyst is particularly suitable for impregnating a particulate filter: - it has a better thermal resistance than a catalyst based on zeolites exchanged with iron (it must indeed undergo damage without any regenerations) periodicals of the filter at very high temperature), - the combustion of soot by N0 ₂ (effect called "CRT"), at a temperature of about 250 ° C to 350 ° C, tends to reduce the ratio NQ> / NOx, the formulations to base of zeolites exchanged with copper (Cu) being also better adapted because they are less sensitive to low temperature than those exchanged with iron, - it also has a higher storage capacity of NH ₃ . This last characteristic is particularly interesting because the small volume (the short length for an unchanged section) of the SCR organ can be the cause of NH ₃ leakage. It is therefore very useful that these ammonia leaks can be "captured" correctly in the SCRF brick downstream of the SCR member. It should be noted that the copper exchanged zeolites proposed for the SCRF and / or exchanged with iron for the catalyst of the catalytic reduction catalyst member SCR are for example based on zeolites of the chabazite, ferrierite or hydrated aluminosilicate type (ZMS5). ), and may also contain at least one of the following oxides: cerium (Ce) oxide, zirconium (Zr), or at least one of the following metals: niobium (Nb), tungsten (W), titanium ( Ti).

According to one embodiment, the support of the oxidation catalyst member and / or that of the NOX trap member, is optionally equipped (s) with heating means, for example of electric resistors type. This reduces their rise time and therefore the time from which they start. Alternatively, one can use for one and / or the other of these bodies a ceramic-type material such as cordierite.

The support of the SCRF particle filter may be, for example, silicon carbide (SiC), cordierite or aluminum titanate.

Advantageously, the after-treatment device according to the invention also comprises a mixing member of the exhaust gas and the reducing agent and / or the precursor of the gearbox between the mouth of the gearbox introduction means and / or precursor of a reducing agent for the selective catalytic reduction of SCR nitrogen oxides and the selective catalytic reduction catalytic converter of nitrogen oxides integrated in the SCRF particulate filter. This mixer has the function of mixing as well as possible the exhaust with the reducing agent or reducing precursor, this being particularly useful when the precursor is of the liquid type, such as urea in aqueous phase. The invention also applies to the direct injection of the reducing gas, such as ammonia, which feeds the exhaust line from one or more salt cartridges (in particular of SrCI ₂ type) capable of adsorbing ammonia and releasing it by thermal activation, in a known manner (technology commonly called "solid" SCR), and in this case, the mixer is less necessary.

Preferably, the mixer is of a type having a path length for gases passing through it at least twice the length it occupies longitudinally in the envelope. The purpose of the mixer is to homogenize the mixture between the exhaust gas and the reducing agent, and, if a precursor of a reducing agent is introduced, to promote the decomposition of the reducing agent precursor into a reducing agent. The use of a mixer imposing on the exhaust gas a relatively long path compared to the length of the mixer, for example of a type imposing gas a substantially helical path with impactor, is particularly suitable for the invention. It makes it possible, by obtaining an exhaust gas travel distance greater than its own dimensions, to use in a compact device a solution based on urea as an ammonia precursor, even though that the thermolysis of urea corresponding to the transformation under the action of the heat of urea into isocyanic acid HNCO and ammonia NH ₃ ) in the exhaust gas requires a significant time. The mixer may also be, for example, a T-mixer using downstream gas recirculation oxidation catalyst in a double jacket around the oxidation catalyst with an injection on the exit face of the oxidation catalyst.

Preferably, the single envelope is substantially in the form of a cylinder provided with an inlet divergence and an outlet convergent (in the form of cone sections), with a total length of at most 450 mm, especially at most 400 mm, preferably between 280 and 380 mm, and therefore has a compactness quite compatible with an implantation in a sub-bonnet of a motor vehicle.

Preferably, the means for introducing the reducing agent is a solenoid or piezoelectric or mechanical or hydropneumatic type actuator type injector. The duct between the exhaust manifold and the device according to the invention may further comprise one or more turbocharger turbines in the context of a supercharged engine, and, in particular, the device according to the invention may be connected directly to the casing of a turbocharger, at the outlet of a turbine. The invention also relates to an exhaust line which comprises the post-processing device described above.

The invention also relates, in a first embodiment, to a motor vehicle defining a space under the hood, which contains what is usually referred to as the engine compartment, and a space under the box, and comprising a a heat engine connected to the preceding exhaust line, such that the engine and the aftertreatment device of the exhaust line are arranged in the space under the hood. In this way, all the depollution units are grouped compactly close to the engine. The invention also relates, in a second embodiment, to a motor vehicle defining a space under the hood and a space under the body, and comprising a heat engine connected to the previous exhaust line, such as the engine. the DOC oxidation catalyst member or the LNT NOx trap, the mouthpiece, and the particulate filter member provided with a SCRF catalytic selective reduction catalyst coating of the NOx nitrogen oxides of the post-treatment device the exhaust line are arranged in the space under the hood, and such that the ammonia leak treatment member is disposed in the underbody space. This brings together the closer to the engine the pollution control organs needing a high exhaust gas temperature and away from the engine the ammonia leakage treatment member to preserve too severe heat that could cause a degradation of its effectiveness.

The invention also relates to a method of implementing a motor vehicle comprising a heat engine connected to the exhaust line as described above, the treatment of NOx being provided by two systems, d ' on the one hand the NOx trapping member, and on the other hand the SCR / SCRF assembly, and, depending on the availability of the reducing agent on the vehicle, adjusting the NOx treatment weighting between one or the other systems.

It should be noted that, among the different variants of the present invention, the most interesting in terms of NOx emission reduction and NH ₃ management has been found to comprise the following series of organs, presented according to an upstream to downstream sequence:

(NOx trap body / SCRF SCRF / SCR-coated particle / agent mixer); all these bodies being grouped in a common envelope in the engine compartment, especially at the output of the turbocharger when it is present.

The invention is described in more detail below with reference to the figures relating to a non-limiting embodiment relating to a device for aftertreatment of the exhaust gas of a diesel engine:

- Figure 1 shows schematically a motor and its exhaust line of a motor vehicle comprising the after-treatment device according to an example 1 of the invention;

FIG. 2 is a graph showing the evolution of the ammonia storage capacity in the SCRF member as a function of the temperature;

FIG. 3 represents an operating diagram of the ammonia leak treatment catalyst of the post-treatment device of FIG. 1.

The references taken from one figure to another designate the same components, and the various components shown are not necessarily scaled. The figures remain very schematic to facilitate reading.

In the invention, and as shown in Figure 1, there is provided a device for treating the exhaust gas of a motor 1 according to an example 1. This device is integrated in the exhaust line connected to the manifold (not shown) of the exhaust gases of the engine 1. It comprises, in the same envelope 2 (which can also be referred to as the English word "canning") and, depending on the direction of flow of the exhaust gas (upstream then downstream) a catalyst member d 3 or a NOx trap 3 'according to one or the other embodiments of the invention, a mouth 41 of a means 4 for introducing a reducing agent (or a reducing agent precursor). ), a mixer 5, a particulate filter SCRF provided with an impregnating coating SCR 7 and a catalyst for treating ammonia leaks 8. At least one NOx sensor 9 is also provided between the filter 7 and the catalyst ammonia treatment 8.

The casing 2 is located closer to the exhaust manifold, in particular about 300 mm from its outlet (for example at most 500 mm from its outlet). It is arranged, in the motor vehicle, in the under-hood space accommodating the engine 1. The dimensional / geometric data are as follows: The casing 2 is of cylindrical section and accommodates the various members 3,7 and 8, also of substantially cylindrical outer shapes and sections of about 175 cm ² - ( or 150 mm in diameter). The ends of the casing 2 are in the form of cone sections, to allow connection to the rest of the exhaust line of much smaller section. The length L12 of the particle filter 7 is between 5 and 8 inches, ie between 127 and 203.6 mm, for example 6 inches or 152.7 mm.

The length L3 of the oxidation catalyst member 3 or NOx trap 3 'is about 70 mm. The length L0 from the upstream face of the oxidation catalyst 3 or the NOx trap 3 'to the downstream face of the member 8 is between 280 and 380 mm. This length corresponds substantially to the length of the cylindrical portion of the casing 2. The total length LT of the casing 2, including the two connecting cones is a little higher. According to a first embodiment (example 1), the first "brick" of this post-treatment device is an oxidation catalyst 3, which oxidizes the reducing species that are carbon monoxide (CO) and the unburned hydrocarbons (HC). The reactions he favors are as follows:

CO + ½ 0 ₂ -> C0 ₂ (R1) Oxidation reaction of carbon monoxide

C _x H _y + (x + y / 4) 0 ₂ -> x CO ₂ + (y / 2) H ₂ O (R 2) Oxidation reaction of unburned hydrocarbons

It consists of a cordierite-type honeycomb support on which is deposited a catalytic active phase ("washcoat") containing precious metals to catalyze the oxidation reactions of CO, HC and NO. This phase also comprises oxides such as alumina doped with various stabilizers (lanthanum, cerium, zirconium, titanium, silicon, etc.). On these oxides, precious metals (platinum, palladium) are deposited in order to catalyze the oxidation reactions at low temperatures. Acidic compounds such as zeolites are also added. Their ability to store hydrocarbons at low temperatures and remove them from storage at high temperatures can improve the treatment of HC during cold phases. To these functions can be added (oxidation of carbon monoxide and unburned hydrocarbons and storage of the latter at low temperature) a storage function of nitrogen oxides, NOx also at low temperature. This storage function is ensured by the introduction of materials of simple or mixed oxides type basic type such as, for example, cerium oxides or barium among others. The injector 4 of urea or the mixer 5 (also called mixing box), already described and known, in particular from the aforementioned patent application WO 201 1/089330, is not described in detail here. Just remember that the mixing box 5 fed by an injector 4, itself fed by a gauge-pump module that draws urea in aqueous solution in a tank of about 20 liters (it may contain less because the volume of urea embedded depends on the consumption strategy adopted), ensures a mixture between the drops of urea and the exhaust gas sufficient for the reaction (R3) of thermolysis to be completely and the reaction (R4) d hydrolysis takes place in part. The reactions (R3) and (R4) are explained below.

The SCRF particle filter 7 treats the nitrogen oxides. The principle of reducing these NOx by SCR (by the coating of the particle filter 7) can be broken down into two main steps:

1> Formation of the reductant (NH ₃ ) from Adblue ® which is a mixture of 32.5% urea and water

(NH ₂ ) ₂ CO -> NH ₃ + HNCO (R 3) thermolysis of urea HNCO + H ₂ 0 -> NH ₃ + CO ₂ (R 4) hydrolysis of isocyanic acid

The decomposition of the urea, injected by the injector 4 into the mixing box 5, is in two stages: a first called "thermolysis" which forms a molecule of NH ₃ and an isocyanic acid molecule ( HNCO) and a second which forms the second molecule of NH ₃ from the hydrolysis of isocyanic acid. These two steps, but especially the vaporization of the water contained in the mixture, require temperatures of at least 180 to 200 ° C, hence the interest that the injecfeur and the gas-liquid mixing box (urea) ) are close to the output of the engine 1. This step makes it possible to form the reducer essential for the operation of the SCR reduction.

2> Selective catalytic reduction of NOx by NH ₃ by the SCR coating of the 7: 4 organ NO + 0 ₂ + 4 NH ₃ 4 N ₂ + 6 H ₂ 0 (R 5) SCR standard NO + N0 ₂ + 2 NH ₃ ^ 2 N ₂ + 3 H ₂ 0 (R6) SCR with fast kinetics

6 N0 ₂ + 8 NH ₃ - »7 N ₂ + 12 H ₂ 0 (R7) SCR with slow kinetics

Several reactions can take place (R5 to R7), but the optimal and sought conversion of NOx is obtained thanks to the reaction (R6) whose kinetics is the fastest but whose stoichiometry imposes a close ratio N0 ₂ / NOx 0.5, especially at low temperatures (that is, at most 250 ° C).

The SCR catalyst of the particulate filter 7 is based on copper zeolites, such as chabazite, β, copper-ferrierite, ZSM5 ... This is the best choice, especially for the catalyst of the filter to particles remains effective even at very high temperature (that it resists the regenerations of the filter in particular). The porous support of the filter 7 is rather of SiC silicon carbide.

Obtained with the architecture of this example 1 according to the invention favorable thermal conditions, which results in a high efficiency in the treatment of NOx: it was measured on a WLTC cycle at the exit of the exhaust line a level of 40mg NOx / km for a standard at 80mg / km. Thus, with the invention, a very low level of NOx emissions to the atmosphere is obtained, while preserving the compactness of the entire post-treatment device.

Returning now to the operation of ammonia leakage treatment catalyst 8. In Example 1 discussed, we chose a catalyst member 8 of ASC type, whose operating principle and structure are illustrated in Figure 3. It has two impregnation layers: a layer C2 which performs the oxidation function of NH ₃ in NOx and a layer C1 which performs the NOx reduction function by NH ₃ .

The composition of the ASC 8 member is thus as follows: the upper layer C1 (that which is in contact with the exhaust gas) corresponds to a catalytic coating of the SCR type and the lower layer C2 (that which is in contact with the walls of the substrate contains precious metals (preferably palladium in a very small amount, between 0.5 and 5 g / ft 3, ideally from 1 to 2) deposited on alumina. the following: the residual ammonia enters the layer C1 and stores itself in this layer in part.The remainder of the ammonia passes through this layer C1 and enters the layer C2 whose precious metals (Pd) favor the oxidation of the ammonia NH ₃ to NOx. When the NOx apparent from the SCR catalytic coating of layer C2, they necessarily fro by the C1 layer which is stored NH _3. the NO ₃ reduction reaction by NH ₃ can then take place. The NOx are thus converted into nitrogen (N ₂ ) before emerging from this catalyst 8.

Exiting the exhaust line, there is no, or almost no risk of ammonia emissions into the atmosphere with this catalyst 8, even if the engine team a utility vehicle with a strong need in NOx treatment, as explained below:

If we take all the reactions R3 to R7 explained above, it is thus understood that there is a storage phase of NH ₃ in the catalytic coating of the filter SCRF 7, prior to the conversion of NOx into N ₂ thanks to this same NH ₃ generated by the decomposition of urea. This ammonia storage represented by a criterion called "NH ₃ storage capacity" evolves according to various parameters such as the intrinsic characteristics of the catalytic coating, its state (new or aged), the intra-catalyst temperature and, to a lesser extent , the residence time of the gases in the catalyst ... There are, thus, SCR catalytic coatings which have more or less NH ₃ storage capacities: the ammonia storage capacity of the SCR coatings to For example, copper base is higher than that of iron-based coatings.

For a given SCR catalytic coating, its aging state will intervene: the older the coating, the more it will lose some of its storage capacity. For a catalytic coating SCR and a given state of aging, the storage capacity will evolve as a function of the intra-SCRF temperature, as shown in FIG. 2. This figure represents a graph, with, on the abscissa, the temperature in degrees. C, and, ordinate, the mass of NH ₃ stored by SCR coating in milligrams / liter. . On reading this graph, we observe a drop in storage capacity with temperature, a decrease due to the phenomenon of thermodisorption of ammonia. Finally, it can be noted that the oxidation of NH ₃ to NOx occurs from about 450 to 500 ° C.

It is thus understood that there are phases where emissions of NH ₃ to the cannula (that is to say at the downstream end of the exhaust line) can appear: - the NH ₃ can be destocking the SCR coating of the filter 7, because of the thermal conditions encountered, without reacting with the NOx; the NH ₃ can no longer be stored for reasons of saturation of the storage sites (especially if the SCR coating which has aged thermally lost storage capacity); - or if the amounts of NH ₃ are too large compared to its storage capacity.

The pollution control systems are generally effectively controlled by on-board diagnostic tools known as "OBD" for "On Board Diagnosis" throughout the life of the vehicle. Thus, there are various procedures that make it possible to check whether the pollution control systems are in working order: - by sub-injecting urea and therefore ammonia, the system must be able to detect a lower efficiency of NOx treatment; or, on the contrary, by over-injecting urea and specifically creating NH ₃ emissions downstream of the SCRF filter 7, the system must be able to detect them. In the latter case, the system is diagnosed using the NOx sensor 9 located downstream of the SCRF filter 7. The present invention eliminates these downstream ammonia leaks, by adding the organ 8 of the treatment of ammonia, and thus to the cannula, at the end of the vehicle exhaust line. The heavier the vehicle, the more these phases at risk (urea injected quantities very important to "respond" to the amounts of NOx produced by the engine, reduced NH ₃ storage capacity due to the high thermal conditions, etc. ..) can occur and be responsible for NH ₃ emissions to the cannula, even with a line of pollution as effective as mentioned above. Alternatively to the ammonia treatment unit 8 of the ASC type, it is possible to use a CUC type catalyst, which has a unique function of oxidizing the NH ₃ to NOx, thanks to a catalytic coating containing platinum. (The Pt load is not necessarily important: 10 to 20g / ft ³ may be sufficient). It is a simple solution but less efficient than the previous one, since it will recreate NOx at the end of the exhaust line. It is then necessary to ensure that the sum of the NOx not treated by the filter SCRF 7 and residual emissions of NH ₃ converting to NOx by the action of such a catalyst of the CUC type does not exceed the limit of NOx emission regulation.

According to another variant not shown, positioning the ammonia leakage treatment catalyst 8 further downstream on the exhaust line: it is no longer in the common envelope 2, closer to the engine in the space under the hood, but downstream in the underbody space, in a dedicated envelope further on the line. This implantation is of interest to avoid premature thermal aging of the catalyst for treating ammonia leaks 8. According to another embodiment, (Example 2), the oxidation catalyst is replaced by a NOx trap 3 '. It consists of a cordierite-type honeycomb support on which is deposited a catalytic phase comprising storage promoting elements such as, but not only, simple or mixed oxides of barium and / or magnesium.

In lean mixture, the NOx trap will store the NOx by carrying out two reactions in series:

NO +1/2 0 ₂ -) N0 ₂ 2 N0 ₂ + ^" Ba ^" -) Ba (N0 ₃ ) ₂

In a rich mixture, the NOx trap destocks and processes at least some of the NOx by carrying out two other series reactions:

Ba (N0 ₃ ) ₂ -) NOx + "Ba"

NOx + "reducers" (HC, CO ..) -) N ₂ [0087] The other organs downstream of the NOx trap are the same as in example 1 above.

In conclusion, thanks to the post-processing device of the invention, it is not only possible to meet the increasing requirements of future standards, particularly with regard to NOx emission levels, but it is also possible to reduce fuel consumption through the design and / or control of the combustion engine, the C0 ₂ / NOx compromise towards "low C0 ₂ " strategies, while reducing the ammonia emissions at the end of the exhaust line.

Claims

Device for the aftertreatment of the exhaust gases of a combustion engine (1), characterized in that it comprises, from upstream to downstream:

A DOC oxidation catalyst member (3) or a NOx LNT trap member (3 ');

• a mouth (41) of means (4) for introducing a reducing agent or precursor of a reducing agent for the selective catalytic reduction of SCR nitrogen oxides;

A particulate filter member (7) provided with a catalytic selective catalytic reduction coating SCRF of nitrogen oxides NO _x ;

• an ammonia leak treatment unit (8).

2. post-treatment device according to the preceding claim, characterized in that the DOC oxidation catalyst member (3) or the NOx trap member LNT (3 '), the mouth (41), and the particulate filter member (7) provided with a catalytic selective catalytic reduction coating SCRF NOx nitrogen oxides are grouped in a single envelope (2), and in that the organ of ammonia leakage treatment (8) ) is disposed outside of said single envelope.

3. post-treatment device according to one of the preceding claims, characterized in that the action of the ammonia leak treatment member (8) is completed by an ammonia leak treatment coating integrated in the particle filter (7), preferably in its downstream part.

4. post-treatment device according to claim 1, characterized in that said organs and mouthpiece are grouped in a single envelope (2).

5. post-treatment device according to one of the preceding claims, characterized in that the ammonia leak treatment member (8) is a catalyst for treating ammonium ammonia leakage by ammonia oxidation in ammonia. NOx then the reduction of said NOx into nitrogen.

6. post-treatment device according to one of claims 1 to 4, characterized in that the ammonia leak treatment member (8) is a catalyst for cleaning CUC ammonia leaks by oxidation of the ammonia to NOx.

7. post-treatment device according to one of the preceding claims, characterized in that the oxidation catalyst member (3) comprises a nitrogen oxide adsorbing material PNA.

8. Exhaust line, characterized in that it comprises the aftertreatment device according to one of the preceding claims.

9. A motor vehicle delimiting a space under the hood and a space under body, and comprising a heat engine connected to the exhaust line according to the preceding claim, characterized in that the engine and the aftertreatment device of the exhaust line are arranged in the space under hood.

10. A motor vehicle delimiting a space under the hood and a space under body, and comprising a combustion engine connected to the exhaust line according to claim 8, characterized in that the engine, the oxidation catalyst member DOC (3) or the NOX LNT trap (3 '), the mouth (41), and the particulate filter member (7) provided with a SCRF catalytic selective reduction catalyst coating of the NOx nitrogen oxides of the post-injection device. treatment of the exhaust line are arranged in the space under hood, and in that the ammonia leak treatment member (8) is disposed in the underbody space.

11. A method of implementing a motor vehicle comprising a heat engine connected to the exhaust line according to claim 8, characterized in that the treatment of NOx is provided by two systems, on the one hand the trap member NOx (3 '), and on the other hand the particle filter member (7) and that, depending on the availability of the reducing agent on the vehicle, the NOx treatment weighting is adapted between the one or the other systems.